Method of making wafers with disseminated particles



Jan. 21, 1964 B. TUROVLIN ETAL 3,118,221

METHOD OF MAKING WAFERS WITH DISSEMINATED PARTICLES Filed Feb. 26, 19593072' 03 AI PARTICLES PARTICLES PUT IN EVACUA TE CAN TO I-M/CRON a SEALHEAT 20o TON EXTRUS/ON PRESS j u RAM .6/5

600 f ROD DIA.

BERNARD TUROVL/N JAMES R. LINDGREN IN V EN TORS y 560.445, Egwedtkmfi-ATTORNE Y3 United States Patent 3,11%,221 ETHGD 0F MAKENG WAFERS WITHDISSEIVIINATED PARTHZLES Bernard Turovlin and James R. Lindgren, denDiego, Calif, assignors to General Dynamics Corporation, New York, N.Y.,a corporation of Delaware Filed Feb. 26, 1959, Ser. No. 795,715 1 Claim.ct. 29-4205) The invention relates to the manufacture of a dispersion ofa functional ingredient in a solid metal vehicle wherein the functionalsubstance is distributed through the solid carrier metal not in the formof an alloy, but in particles disseminated through and spaced apart fromeach other in the carrier metal.

The functional substance is preferably disseminated through the carriermetal in small particles. For certain purposes, the particles may bemade quite fine, for example, 325 mesh, or having a maximum particlediameter size of from to microns, or, for some purposes, the particlesize may be much larger. Also, depending upon the use, the functionalsubstance may weigh as much as about 15 percent of the weight of thecarrier metal, whereas in other cases, the percentage of functionalsubstance may be much lower, for example, 1 percent of the carrierweight.

Preferably, a product made in accordance with our process has a body ofmalleable metal or metal alloy powder which may be worked or formed intovarious sizes, shapes or thicknesses, depending upon the purpose forwhich the product is ultimately to be used. For example, the process maybe used in the manufacture of such articles as abrasive elements ortools such as files or grinding wheels. One important application of theinvention is in the field of nuclear physics, and particularly innuclear reactors, where pieces of the material may be employed ascomponents of or adjuncts to the fuel core or assembly.

Although the metal aluminum is particularly well suited for use as thecarrier, in many cases other metals, for example, lead, magnesium,copper, nickel, iron, ferrous metals or alloys of the same with othermetals, can be successfully employed, depending upon the purpose forwhich the final product is to be used. However, it is im portant thatwhatever metal or alloy be selected as a carrier, it should be capableof self-welding when subjected to a pressure and heated to a temperautresuch as can be reasonably well attained in extrusion equipment.

Products made by our process generally are used for purposes whichrequire that the particles of the functional substance be spaced apartfrom each other, and that that spacing should be maintained during theuse of the product. Hence, not only should the particles be properlydisseminated or distributed through a mass of small particles of thecarrier metal during the initial or mixing stage of the process, but inthe latter stages of the process, migration of the particles within themass to form clustters or aggregates should be prevented as far aspossible.

Although products of the general class described may be made by othermethods, we prefer to use an extrusion method in which the carrier metalor alloy n eds to be heated to a temperature substantially less than thenormal fusion or melting temperature of the metal alloy, butsuificiently high so that when the material, in comminuted form, isextruded under heavy pressure, the particles of the metal will be, ineffect, welded together to form a solid, homogenous metallic mass. Thefunctional substance, also in particle form, which has been disseminatedthrough the mass of metal particles by a suitable milling or mixingoperation prior to the insertion of the mass into the extrusionequipment, according to our method should be of a nature that it willnot alloy with the carrier metal 2 at the pressure and temperaturecondition of the press so that the discrete particles of the substancewill not form an alloy with the particles of the carrier metal and willnot migrate and form undesirable accretives or nodules composed or twoor more particles of the substance.

We are aware that others have heretofore proposed to produce or haveproduced what may be called self-welded metallic bodies made frommetallic particles in an extrusion press under heavy pressure attemperatures below the fusing point of the metal, and that suchproposals have involved the production of alloys by the use of mixturesof particles of diiferent metals.

The drawings accompanying this description illustrate a specific exampleof the invention as used in the manufacture of a malleable metallicproduct from which can be made a disc or wafer containing as afunctional substance a small amount of Samarium oxide (Sm O aluminummetal being used as a carrier. Such a disc or Wafer may be effectivelyemployed as an adjunct or component of the fuel system of a nuclearreactor.

In the drawings:

FIGURE 1 is a plan view of the disc or wafer;

FIGURE 2 is a section taken on the line 22\ of FIGURE 1;

FIGURE 3 is a diagrammatic view of a flow sheet showing the process forproducing an extruded rod;

FIGURE 4 is a diagrammatic view showing the process of cross-rolling oneof the rod sections; and

FIGURE 5 is a plan view of the cross-rolled sheet after the discs havebeen punched therefrom.

Example The carrier in the present instance is a metal of low neutroncross section, which is preferably capable of being cold-Welded whenreduced to small particle size. Aluminum is such a metal. By means ofconventional methods not necessary to describe here, the aluminum metalis prepared in finely divided form. Preferably, at least approximately99 percent of the mass has a particle size less than 325 mesh (.044mm.).

The functional substance in the specific example described is samariumoxide (Sm O- This is also converted to fine particles by conventionalequipment or methods so that 99 percent of it will go through a 200 meshscreen (0.074 mm). Samarium metal melts at 1350 C. (2440 F), and theoxide cannot be welded at a temperature of 1000 F. at the pressure weemploy.

The aluminum and Samarium oxide may be charged alternately and inseveral increments into a Patterson- Kelly Model 1646 Twin-ShellBlender. The powder may be used in the as received condition, and insuch proportion that at the conclusion of the mixing operation, the masscontains approximately 116% by weight of Sm O powder. After mixing hasbeen carried on for a period of one hour, the mass is sent through aWeber Brothers Laboratory lulverizing Mill (.008 screen), whicheliminates aggregates or conglomerated particles.

The powder mixture is then canned by charging it into a hollow cylindermade of aluminum of 35 grade having an external diameter of 3 /2", witha 9.216 wall to which end plates /4" thick of aluminum of 38 grade maybe welded by the heliarc process. One end of the can is equipped with a/8 0.1)., A 1D. aluminum tube for evacuation. All components of the canare thoroughly cleaned prior to canning. After evacuating for threehours to a pressure of approximately one micron, the evacuation tube issealed.

The filled and evacuated cans are then placed in a billet furnace andheated to a temperature of 1000" F. (The fusion temperature of aluminumis about 658 C. or

1220 F.) This temperature of 1660 F. is held for three and one-halfhours prior to extrusion.

The extrusion operation is performed as soon as possible after the canis removed from the billet furnace. The extrusion press may be of anyconventional type, and preferably is of such size that the chamber cansnugly receive the can which is preferably located in line with the axisof the ram. The ram may be of the same diameter as the external diameterof the can, i.e., 3 /2" diameter, so that the extrusion ratio works outat about 31.4 to 1 when using a die having a diameter orifice, and iscapable of exerting a pressure of at least 200 tons.

Preferably the press is heated to such a temperature that the die inwhich the extrusion orifice, of a diameter of /8", is located, maintainsa temperature of about 600 F., or more. To facilitate the extrusionoperation, the charge is forced into the die through a conical throat,the walls of which are inclined to the axis of the extrusion chamberabout 45, so that the apical angle of the cone is about 90. Also, tofacilitate the operation, the die preferably is lubricated by anyconventional material or substance normally used in extrusion processes,for example, the compound sold under the designation Spray- Graphmanufactured by American Resin Corporation, Chicago, Illinois, andconsisting largely of trichlorethylene and graphite powder dispersion.

During the extruding operation, the can serves as a means for containingor retaining the powdered mixture within the can when the filled can isbeing equeezed through the orifice of the extrusion press. Obviously theshell of the can is also squeezed out into a shell which is thinner andof smaller diameter than the shell of the can, and serves as a claddingskin on the outside of the rod-like welded mass which is forced out ofthe extrusion orifice.

The resulting rod, when using a can having a length of 7%", was about135" long and 0.615 in diameter. Although the temperature in the can andthe temperature of the press were both considerably below the meltingpoint of aluminum, the pressure, notwithstanding the relatively lowtemperature, was of such order of magnitude, as in the case of othermetals when subjected to such treatment, that it served in effect tocold weld the particles of aluminum together to form a concrete masswithout actually causing complete liquefaction of the aluminum. Althoughthe welding effect apparently results in a complete integration of theparticles of aluminum, the particles of Samarium oxide apparentlymaintain their spaced relationship in the mass, and do not coalesce toform larger aggregates. Any significant change in the spacing oruniformity of distribution of the mass is not apparent. Furthermore, therod appears to have the physical characteristics of a solid rod ofaluminum metal, although the individual particles of samarium oxide canbe detected by microscopic examination.

After trimming off the ends of the rod, the remainder of the rod is cutinto sections or parts six inches long and each of these six inchlengths is cross-rolled in a cold condition to a thickness of .048 inch.This crossrolling can be accomplished in any conventional roll mill. Inthis case, a S x 10 Stanat TA-625 mill may be used, with a reduction inthickness of from 1520% per pass. The rod or strip need not be annealedat any time during rolling.

The resulting sheet from each six inch length after rolling isapproximately six inches by six inches. A width of approximately 73 isdiscarded from the front and rear edges of the sheet, since they containa large fraction of the outer cladding. Then, by means of a conventionaldie, there are punched from the sheet 12 discs, each having a diameterof 1.415. Each disc weighs about 3 grams. The discs may be placed at theouter ends of the fuel rods in the core of the fuel system.

Particularly in the case of the specific example herein described, anevacuation step is important or desirable for preventing bubbles of airor other gas from forming within the mass during any subsequent step ofthe process.

In view of the fact that the metal aluminum in the specific example hasno technical purpose except as a carrier for the particles of samariumoxide, the functional substance which is disseminated through the weldedmass of aluminum, it is not necessary to remove the aluminum skin fromthe extruded rod. However, if it is desired to use the process for themanufacture of such articles as abrasive elements, at the conclusion ofthe operation of working the rod into different shapes of elements, andif it is desired to expose the particles of functional substance on theexterior of the element, the outer skin of cladding metal may be removedchemically, electrolytically or by any other conventional manner.

Various features of the invention are set forth in the appended claim.

We claim:

The process of making solid wafers containing disseminated particles ofsamarium oxide in sufficient amount to enable the wafers to function asuseful components of the fuel system of a nuclear reactor, whichcomprises making a mixture which includes a major percentage of finelydivided aluminum and a useful percentage of Samarium oxide of a particlesize of about 200 mesh distributed through said mixture, sealing saidmixture in a metallic envelope in vacuo, heating the sealed mixture to atemperature below the fusion temperature of aluminum, then extruding themass through a die orifice at a temperature also below said fusionpoint, but at a suificiently high pressure to weld together theparticles of aluminum so as to form an extruded, malleable rod, thenpropelling a section of said rod through a series of pressure rollsuntil the thickness of the material is reduced so as to form a sheethaving a predetermined concentration of samarium oxide per unit of area,and then cutting said sheet into sections, While maintaining thetemperature of the mixture throughout the process below that at whichthe aluminum will fuse or form an alloy with the Samarium oxide, so thatthe particles of Samarium oxide will retain their separate identity andtheir disseminated state throughout the mass of aluminum.

References Cited in the file of this patent UNITED STATES PATENTS2,290,734 Brassert July 21, 1942 2,727,996 Rockwell et al Dec. 20, 19552,805,473 Handwerk et al Sept. 10, 1957 2,843,539 Bornstein July 15,1958 2,859,163 Ploetz Nov. 4, 1958 2,866,741 Hausner Dec. 30, 19582,885,287 Larson May 5, 1959 2,917,821 Fritsch Dec. 22, 1959 2,922,223Boughton et al Jan. 26, 1960 OTHER REFERENCES Positive Control RodsWorth of Some Rare Metal Oxides. See Nuclear Science Abstracts, November15, 1959, Item 19, 699, page 2641.

